This invention relates to a method of sulfur purification. More particularly, it relates to a method of enhancing the purity of sulfur by removing trace metal impurities such as mercury, copper, lead, zinc, iron and cadmium from the elemental sulfur which has been recovered by hydrometallurgical processes such as direct pressure leaching of zinc concentrates. Most particularly, this invention relates to a method of removing a trace amount of, or up to 100 ppm of mercury contained in sulfur, particularly elemental sulfur recovered by direct pressure leaching of zinc concentrates to such an extent that the mercury contact of the sulfur becomes 1 ppm or less.
Impurities in sulfur have conventionally been removed by various methods depending on the type of the impurities; solid impurities such as ashes are removed by precipitation or by filtration using filter aids, and other impurities such as hydrocarbons and tar are removed by adsorption on adsorbents such as diatomaceous earth, silica gel and activated carbon. However, for removal of metal impurities such as mercury, copper, lead, zinc, iron and cadmium that are contained in sulfur in trace amounts (less than a thousand ppm), those methods are not effective and techniques for enhanced removal such as distillation are required. Furthermore, when sulfur is to be used as a raw material for the production of sulfuric acid, the content of mercury in sulfur has to be lowered to 1 ppm or less and, to this end, a more effective method of mercury removal is necessary. This is because mercury is extremely harmful to the human body.
A direct pressure leaching system is drawing increasing attention as a new hydrometallurgical approach to process zinc concentrates. According to this system, the residue obtained by pressure leaching is subjected to flotation and elemental sulfur accompanied by unreacted sulfides is recovered as a float, which is melted at a temperature of 130.degree.-150.degree. C. and filtered, whereupon elemental sulfur is recovered in the filtrate. The sulfur recovered by this method contains metal impurities such as mercury, copper, lead, zinc, iron and cadmium that are each present in an amount of the order ranging from 10 to 10.sup.2 ppm. The sulfur from which such metal impurities have been removed has a purity of less than 99.99%, which is too low to impart market competitiveness to the sulfur for use as raw material in the production of sulfuric acid and in other industries. Hence, a strong need exists to develop a more effective method for removing metal impurities from the recovered sulfur. It is particularly important to remove mercury from the recovered sulfur, because mercury is very harmful to the human body and the recovered sulfur obtained in the pressure leaching of zinc concentrates is usually used as a raw material in the chemical industry, including the production of sulfuric acid and various other engineering and fine chemicals.
From the viewpoint of initial investment and energy cost, distillation and other conventional methods for enhanced removal of metal impurities are by no means considered to be effective in the case where sulfur is to be used as a raw material in the chemical industry including the production of sulfuric acid and various other engineering and fine chemicals.